Black toenails (subungual hematomas) are bruised toenails that result from blood collecting under the nail plate. It’s the result of trauma to the nail area (like being stepped on, or dropping a heavy object on the toe) that causes blood to accumulate under the nail. The pool of blood produces pressure and often severe, throbbing pain. A subungual hematoma can be removed by your podiatrist by making a hole in the nail to drain the blood and relieve pressure, this procedure should not be attempted at home.
Runners frequently suffer from black toenails due to the big toe rubbing against the tip of an ill-fitting sneaker. These nail injuries are generally not painful and will heal as the repetitive trauma ceases. A shoe with adequate toe room will also help in most cases. Although black toenails can be painful and tender, treatment is usually not necessary unless there is bone or tissue damage, too.
A fungal infection can also cause the nail to thicken and appear almost black. This can be treated by your podiatrist with oral or topical anti-fungal medications.